(1) Field of the Invention
The present invention relates to a device for damping the vibration of a structure. More particularly, the invention seeks to damp the vibration of a structure of a rotary-wing aircraft, in particular the cabin of a helicopter.
(2) Description of Related Art
By their nature, helicopters are subjected to high levels of vibration generated mainly in natural manner by said rotary wing. The various vibrations that exist degrade the comfort of the crew and passengers and also the state of parts and equipment of the helicopter, in particular parts and equipment located in the cabin of the helicopter.
The force torsor generated on the main lift and propulsion rotor of a helicopter by the rotation of its blades represents one of the main sources of the vibration in question.
In particular, the fuselage of a helicopter is subjected to forces and moments that are generated by the main rotor, at a reference frequency that is equal to the product of the number of blades of said main rotor multiplied by the frequency corresponding to the speed of rotation of said main rotor. The response of the fuselage is very sensitive to the difference between the resonant frequencies of the helicopter and said product.
The vibration as generated in that way is due mainly to the slipstream exciting resonant modes of the structure of the helicopter, and in particular the first lateral bending mode of the helicopter tail. The vibration due to this first lateral bending mode being excited generally corresponds to a frequency of a few hertz and it is extremely troublesome.
In order to remedy that, it is known to provide a passive vibration absorber that is generally implemented in the form of a resonator. Such an absorber is located at positions where it is desired to reduce vibration, with the absorber acting by being resonant at a predetermined frequency so as to reduce the relative vibration at said frequency.
The vibration absorber is in the form of a damper block fastened on a support such as a spring blade. One end of the support is held fixedly in an orifice formed in a suitable location of the helicopter structure, e.g. in the cabin.
When the structure is subjected to vibration, the absorber is thus excited by the structure. This results in vibration of the damper block, thereby generating a dynamic force that can, where appropriate, oppose the original vibration of the structure.
In order to optimize the efficiency of the absorber, it is appropriate to adjust the resonant frequency of the absorber so that said resonant frequency is equal to the frequency of the vibration of the structure that is to be damped.
Such an absorber may then be adjusted by adapting the mass of the damper block, e.g. by adding washers to the block or by removing them from it. The absorber may also be adjusted by varying the distance between the block and the fixed end of the support, for example by causing the block to slide along a groove formed in the support. The material of the support and its shape also have an influence on the resonant frequency of the absorber.
The absorber may be adjusted by positioning the helicopter on a vibrating bench driven at the reference frequency. A first sensor measures the effect of the vibration, in other words acceleration, where the support is held fixedly, and a second sensor measures the acceleration at the damper block.
The phase offset between those two accelerations is measured, which phase offset should be equal to 90° when the absorber is tuned. If it is not tuned, then the setting(s) of the absorber is/are modified in successive approximations in order to come as close as possible to said desired value of 90°.
It is therefore necessary to perform a considerable number of tests on the vibrating bench in order to obtain a satisfactory adjustment, with the number of tests being particularly large when the accuracy required for the phase offset is high.
Document FR 2 862 392 describes a method of tuning a vibration absorber on a reference frequency, in which method a measurement step is implemented followed by an adjustment step.
Independently of the method used for adjusting the absorber, it can be understood that it may be advantageous to adjust an absorber several times during its lifetime.
The initial adjustment of an absorber might not be optimized in certain stages of flight, e.g. because of aging of the structure fitted with the absorber, because of a modification to the structure, or indeed because of a change in the speed of rotation of the main rotor, in particular for a helicopter.
Document FR 2 739 670 teaches an improvement to a vibration absorber of the above-mentioned type. The damper block is made up both of a main mass that is fastened to a spring blade that is held fixedly at one end, and also of an auxiliary mass of position relative to the support that can be determined by an electric actuator such as a stepper motor. The actuator is controlled by a computer that servo-controls the position of the auxiliary mass on the basis of the phase offset between the two accelerations.
Such servo-control is relatively expensive since it requires special members.